NMR techniques have grown extensively over the past forty years, most notably in the medical instrumentation areas where in vivo examination of various parts of the human body can be seen and in clinical research laboratory uses. In addition there has been some use and interest in the application of these techniques to industrial instrumentation and control tasks. The present invention enables effective utilization (technically and economically) of pulsed NMR techniques in industrial areas to replace or complement existing optical and radiant energy-based instrumentation.
The following is a brief review of NMR theory and concepts pertinent to understanding the present invention. The term, magnetic resonance imaging, or MRI, used below, is an alternative name for NMR. NMR signal is more easily understood by the human eye and brain. Approximately one third of the elemental isotopes and certain compounds with non-zero spin quantum numbers are magnetically active and suitable for MRI detection.
In a simplified model, a spinning isolated nucleus will align itself either with or against a static magnetic field. There will be a nearly equal number of nuclei aligned in each direction since there is only a small energy difference between these two states so a thermal equilibrium exists between these two states. However, statistically there will be a small number of excess nuclei in the lower energy state. It is these excess nuclei which give rise to MRI signals. The term "nuclei" will subsequently refer only to magnetically active nuclei.
Nuclei in a magnetic field will precess similar to a spinning top, because there is an angular acceleration produced by the interaction of the magnetic field and the magnetic moment of the nuclei. This precession occurs in the direction of the magnetic field. Quantum mechanics shows that only a selected number of possible alignments is possible. The precessional frequency is determined by which alignment occurs and the magnetic properties of the nucleus being studied. The fundamental MRI signal is derived from inducing transitions between these different alignments. This is often done by using the magnetic component of an applied RF (radio frequency) signal. When this component is applied perpendicularly to the magnetic field a resonance occurs at a particular RF frequency where transitions between the different alignments happen. This resonance typically occurs in the Megahertz frequency ranges when a strong magnetic field is used. This field is in the 1 Tesla (10,000 Gauss) order of magnitude (i.e. 0.1-2 T).
The effect of a nucleus bonded in a lattice to other nuclei has a great effect on the resonance frequencies. The effect of the other components and the bonding create secondary effects and shielding which cause the resonance frequencies to be different. In effect the chemical environment affects the resonance frequencies and the signal strength.
Pulsed MRI spectroscopy is one specific technique to which the present invention is drawn. This technique uses a radio frequency burst or pulse which is designed to excite all the nuclei of a particular nuclear species. After the application of the pulse there occurs a free induction decay (FID) curve associated with the excited nuclei. Traditional Fourier Transform analysis generates a frequency spectrum which can be used to advantage in studying the nuclei of interest. The duration of the pulses, the time between the pulses, the pulse phase angle and the use of chemical reactants in the sample are parameters which affect the sensitivity of this technique. These frequency techniques are not easily useable in industrial applications, especially on-line applications.
An object of this invention is an improved measurement system which leads to accurate, fast determination of the types and quantity of the nuclear species of interest.
A further object of the invention is to utilize time domain analysis in achieving such system.
Another object of this invention is its application to the industrial, on-line problems of measuring the controlling processes.
The principal variable of interest is moisture, with distinction between free and bound water in an organic or inorganic substance based on hydrogen nuclei precession analysis. But other parameters can be measured based on hydrogen or other sensitive species including e.g., sodium. It is an object of this invention to accommodate a variety of such measuring tasks.
Another object is to accommodate the dynamics of industrial on-line applications including variations of density, temperature, packing and size factors, friction and static electricity, vibration and frequent, repetitive, cyclic and non-cyclic measurements.
Another object of the invention is to use magnetic resonance techniques in polymer analysis, including density and/or melt index measurement.
Another object is to enhance accuracy and reliability of data obtained.
It is an object of the invention to achieve the necessary practical economies consistent with the foregoing objects.